Copper Sulfide Mineral Flotation Crushed ore, along with water and a lime slurry, is fed into a ball mill. This rotating drum contains steel balls that further crush the ore to a fine powder. When the ore/lime slurry emerges from the mill, it is fed to a rake classifier. Particles that are too large to pass from the classifier are returned to the mill, while the overflow is discharged to flotation cells. Air is injected into the flotation cells, and foaming agents are added, creating a froth. Copper ore particles, due to their relatively light weight, become a part of this froth, while heavier particles, such as iron ore, do not. The copper-rich froth, containing 20 to 40% copper, is then separated from the solution for further processing. Rich copper ore is separated from crude copper sulfide ore by means of a flotation process that takes advantage of the physical (as opposed to chemical) properties of small copper ore particles. To maximize the yield of copper, pH control is necessary in the flotation tanks. The copper ore slurry from the grinding mills is mixed with milk of lime (ground-up limestone) to give a basic pH, pine oil to make bubbles, an alcohol to strengthen the bubbles, and a collector chemical called potassium amyl xanthate.  The xanthate is added to the slurry in relatively small quantities.  Xanthate is a long hydrocarbon (5 carbons) hydrophobic, polar molecule.  Raising the pH causes the polar end to ionize and to preferentially stick to chalcopyrite (CuFeS2) and leave the pyrite (FeS2) alone. Air is blown into the tanks and agitated like a giant blender, producing a foamy froth. The chalcopyrite grains become coated with xanthate molecules and attach themselves to the oily air bubbles which become coated with chalcopyrite grains as they rise to the surface and flow over the edge of the tank. The condition of the froth is directly dependent on pH. The flow rate of lime slurry is therefore regulated to keep the pH within the acceptable range. If pH is too low, iron will be entrapped as well as copper, decreasing the value of the copper ultimately recovered. If too much lime is added, the result is a dilute froth that requires additional concentration in later stages, increasing operating costs and wasting lime. In this manner through a series of steps the copper ore is concentrated to an eventual value of over 28% copper. Hydrophylic waste rock particles do not adhere to the bubbles and drop to the bottom of the tank. The waste material that comes out of the bottom of the tanks at the tail end of this process is called "tailings."